Control interaction analysis of hybrid system with grid-following and grid-forming inverters based on admittance decomposition

In high renewable penetrated power systems, both grid-forming (GFL) and grid-following (GFM) inverters play an important role in maintaining the system stability and economic operation. However, the two kinds of inverters exhibit distinct dynamic characteristics; thus, interconnecting them in a close electrical distance may cause the stability concern. In this paper, the small-signal stability of a hybrid system with GFM and GFL inverters is investigated. Based on the idea of admittance decomposition, this paper first decomposes the overall admittance of two inverters into several sub-admittances corresponding to different control loops and circuit components. Then an in-depth analysis is conducted to reveal the dynamic interaction between multiple timescale control loops based on decomposed admittances. Moreover, the impact of the line impedance, the grid strength, and the power flow on the system stability is analyzed and a security region is further developed. The accuracy and effectiveness of the analysis are validated through eigenvalue analysis, simulations and hardware in the loop based experiments.